The core concept of ultrasound distance detection is to transmit an ultrasound pulse at an obstacle and measure how soon echoes from the obstacle are received in order to determine the distance to the obstacle. Specifically, the time between transmission of the pulse and reception of the echo is linearly proportional to the distance to the obstacle. However, because the echo reduces in amplitude as distances get farther, an increase in distance results in a decrease of reliability because the amplitude of the echo is difficult to distinguish from noise.
One straightforward approach to mitigating this tradeoff is to increase the signal-to-noise ratio by increasing transmit power. However, in some applications, including parking-assist systems, transmit power is effectively capped by other systems. Another approach to mitigating this tradeoff is to increase the length of the transmit pulse. However, longer transmit pulses make the spectrum more narrow, make Doppler compensation more difficult, and weaken time resolution. These negative side-effects are particularly harmful to the utility of rapid-response obstacle-monitoring applications including parking-assist systems.